1. Field of the Invention
The present invention pertains to reflection control coatings for optical elements and to protective coatings for such elements. The invention is adapted to coatings for an optical element constructed of diamond, and is particularly applicable to a coating which is refractory, which is transmissive to infrared, and which reduces unwanted reflection. The coatings may have plural, diverse layers and may include amorphous hydrogenated silicon and aluminum nitride or an oxide of hafnium or yttrium or other refractory oxide.
2. Description of the Prior Art
Conventional materials, such as zinc sulfide and zinc selenide, used for infrared transmitting domes, windows and the like elements, have relatively little resistance to abrasion and fracture. This deficiency is particularly serious in such windows exposed to aerodynamic heating with accompanying particle and raindrop impact. However, diamond is highly resistive to abrasion and thermal shock, is transmissive from the ultraviolet through the infrared, and is resistive to radiation damage. Recent developments provide polycrystalline diamond of optical quality for use as a protective coating on such conventional materials or for use by itself as an optical element. However, diamond has a very high index of refraction, particularly in the infrared, so that reflective losses at an air to diamond interface limit infrared transmission to about 70%. Also, although diamond is highly resistive to environmental effects, diamond oxidizes in air at temperatures of 700.degree. C. or more, conditions typically sustained in aerodynamic heating.
It is, therefore, highly desirable to provide diamond surfaces in optical elements with anti-reflective coatings providing high transmission in the infrared, and it is highly desirable that such coatings also resist oxidation during aerodynamic heating. To be effective, the resistance of diamond elements with such coatings to environmental damage should be comparable to that of diamond itself, and the coatings should not become detached or otherwise damaged by thermal effects including thermal cycling.